In many applications it may be necessary to amplify an analog signal exhibiting a wide amplitude range. For example, a wide range of input signals may be present at the receiving end of a multi-mode fiber optic cable. Such a signal may require analog conditioning or digital signal processing to correct for degradation introduced by the physical medium of transmission, i.e., the optical cable itself.
In many signal conditioning systems, especially communication links, in order to compensate for a wide amplitude range of received information bearing signals, the input signals are subjected to amplitude adjustment using a VGA (“Variable Gain Amplifier”). A VGA allows for the selection and adjustment of gain to be applied to an input signal. Amplitude adjustment or so-called gain adjustment of an incoming signal by a VGA is used to achieve an amplitude level well above the noise and offset thresholds. Without the application of gain adjustment, it may not be feasible to perform further post processing of an incoming signal, such as adaptive equalization.
Cascading gain stages may provide a wide range of amplification and/or attenuation. However, each additional stage may be undesirable as it will introduce harmonic distortion. Harmonic distortion typically arises due to non-linearities inherent in each stage.
Thus, it is desirable to devise an amplitude adjustment scheme using a VGA with a low number of gain stages such that the VGA is suitable for high bandwidth and high linearity applications with a wide amplitude adjustment range.